Ignition system



Jan. 18, SMITH IGNITION SYSTEM v lnneni'or a zzarian E 5m T attorneyFiled Oct. 29, 1945 Patented Jan. 18, 1949 IGNITION SYSTEM Lucian B.Smith, Flint, Mich., assignor to General Motors Corporation, Detroit,Mich, a corporation of Delaware Application October 29, 1945, Serial No.625,245

This invention has to do with high frequency ignition systems forinternal combustion engines and is an improvement on the systemsdescribed and claimed in the following prior patents: No.

2,030,228 granted Februar 11, 1936, to D. W. Randolph and O. S.Dufiendack and R. R. W olfe; No. 2,071,573 granted February 23, 1937, toD. W. Randolph and Hector Rabezzana; and No. 2,197,114 granted April 16,1940, to Hector Rabez zana and W. A. Bychinsky.

The system is characterized by the fact that the rate of voltage rise atthe spark plugs is so high that the plugs will continue to function longafter they would have been short-circuited by carbon, lead or othercombustion chamber deposits had they been supplied with low frequencycurrent. The high frequency discharge is so rapid that there isinsufiicient time for short-cir cuiting to take place.

The system is also characterized by distribution of current to the plugsat low voltage together with the provision of a transformer at each ofthe plugs to raise the voltage to that required to produce the spark inthe engine. By this arrangement diflicult problems arising from highvoltage distribution, such as flashover at high altitude, deteriorationof the wiring harness by Corona discharge, and leakage of currentthrough the capacitance of the ignition cable are avoided. At the sametime the ignition circuit may be much more easily shielded so as not tointerfere with radio communication.

The improved system, like those disclosed in the patents referred to,embodies a condenser in series with a gaseous discharge tube and theprimary of a transformer. The condenser is supplied with energy from asuitable source such as a magneto, preferably having associatedtherewith a low tension transformer. The condenser stores energyreceived from the magneto until the voltage of the tube is reachedwhereupon a high frequency discharge takes place and this, through theplug transformer causes a spark to jump the gap of the plug. In theimproved circuit herein disclosed, but one condenser is employed toserve all of the plugs through a suitable distributor, but if preferredindividual condensers may be used for each plug.

There is also preferably provided in connection with the magneto asuitable breaker which interrupts the current adjacent the peak of thegen erated current and insures accurate timing of the sparks.

An important feature of the invention consists in the use of aresistance in series with the con- 5 Claims. (Cl. 315255) denser. Thisresistance limits the charging current to the condenser so that fewerspark discharges take place through the circuit. Its use makes certainthat the heavy tube current is not flowing at the time that theconnection is broken at the distributor segment and thus increasesdistributor life. It also prevents unnecessary loading of the secondaryof the low tension transformer which might otherwise prevent the fluxfrom building up in the transformer in preparation for the next spark.

Various other features of the invention will be disclosed in connectionwith the following description in which:

Figure 1 is a diagrammatic view of the ignition system.

Figure 2 shows diagrammatically a modified form of resistance for use inthe circuit of Figure 1.

Referring to Figure 1 there is illustrated as the source of energy amagneto of conventional construction consisting of permanent magnets In,each provided with pole pieces l2 and M, the latter being connected bylaminated core 56 on which are wound a primary winding P and a secondarywinding S. I8 indicates a laminated rotatable inductor driven from theengine in any suitable manner not shown. The permanent magnets I0 arepreferably arranged with like poles adjacent the opposed pole pieces I2so that upon rotation of the inductor magnetic flux flows through thelaminated core l6 first in one direction and then in the other, therebyproducing an alternating current in the windings P and S. In series withthe primary winding P are the contacts of a circuit breaker 20 ofconventional construction provided with the usual condenser as shown toreduce sparking. The primary circuit may be provided with a groundconnection as indicated at 22. The secondary winding S ma be, but notnecessarily, connected at one end to the primary circuit as shown and inseries therewith is a resistance R, preferably in the neigborhood ofohms, and condenser C, the latter being grounded to complete thecircuit. A condenser having a capactiy of .1 mf. was found adequate inthe installation.

In some applications, particularly Where radio interference is a seriousproblem, it may be desirable to provide a separate ground for secondaryS of high tension transformer T and, instead of grounding primary P ofsaid transformer, condenser C and secondary S of the magnetotransformer. connect these elements by means of an insulated conductor.This would eliminate the normally heavy grounding currents which flowthrough the engine frame.

Arranged in the discharge circuit for the condenser C is the distributorD. In series with each of the contacts 24 of the distributor is a tube26 and the primary P of a transformer T, associated with one of theplugs. For simplicity there is shown the complete circuit between butone of the distributor segments and one of the plugs but it will beunderstood that in practice a similar circuit is provided for each ofthe segments.

The secondary circuit consists of the winding S connected at one end tothe primary P and to ground as shown and at the other end to the centerelectrode of the spark plug 28, the other electrode of the spark plugbeing grounded as usual. I

The system operates as follows:

When the inductor I8 is driven by the engine the reversing flux set upin the laminated core l6 produces an alternating voltage in thetransformer coils P and S. The engine-driven interrupter 20 is so timedas to break the primary circuit when the generated current is at or nearmaximum value. The resulting collapse of the magnetic field associatedwith the interrupted primary current induces a voltage in the secondaryS which is applied to condenser C through resistance R. The condenserserves to store the energy and, before the charge on the condenser hasreached the critical value, the condenser is connected through thedistributor D to a tube 26 and the primary P of the transformer T. Whenthe condenser voltage reaches the critical value necessary to cause thetube 26 to break down an arc is established between the tube electrodesand a heavy current surge of steep wave front goes through the primary Pof the transformer T, inducing a current of high frequency and highvoltage in the secondary. winding S which in turn produces a dischargeat the spark plug. The condenser C, tube 26and primary P of the sparkplug transformer act' as a high frequency generator and, if thesecondary of the spark transformer is open, the current oscillates. Thefrequency of oscillation in a typical installation was approximatelyone-third to one-half megacycle.

The resistance R limits the charging current to the condenser C so thatfewer discharges take place through the oscillatory circuit consistingof the condenser, tube and primary P. The resistance also serves to dampout the current following discharge at the plug to insure that there isno flow at the time the connection is broken at the distributor and thisinsures longer life for the latter. The resistor likewise limits thecurrent in the secondary S so that no difliculty is encountered inreversing the magnetization of the core in preparation for the nextspark.

The magneto equipped with the breaker as described provides voltagecharacterized by a steep wave front. This characteristic is accentuatedby the condenser discharge through the tube. As a consequence the periodof time during which the spark plug electrodes are subjected to voltagebefore a spark occurs is so short that there can be no important leakageof current through paths in parallel with the spark plug gap such as areoften provided by conducting deposits of carbon; lead, moisture or otherelements on the portion of the insulator projecting into the combustionchamber.

The voltages obtained with this system across such a shunting resistanceare exceptionallyhigh as compared with those obtained with a magnetoignition system of conventional type. Thus, while with the latter sixkilovolts could be obtained across a leakage path having 135,000 ohmsresistance, with the system here described six kilovolts have beenobtained across a leakage path having but 6,000 ohms resistance.

It is characteristic of the conventional ignition stantially constant atall speeds in excess of that required for starting. Thus, in the case ofa particular installation the speed at which the present system began tofunction was approximately 25 R. P. M. of the engine and the voltageobtained at the plug at that engine speed was substantially the same asthe voltage obtained at top engine speeds. Similarly, while in aconventional magneto ignition system the voltage at the plug variessomewhat with the temperature, in the present system the voltage output"is but slightly affected by increased temperature.

Engine tests have indicated that the wearfon spark plug electrodes isconsiderably less with the ignition system here disclosed than withconventional systems. This advantage no doubt results from thesteep-fronted secondary currentan'd the very short time it is effective;

It has been discovered in' tests of engines equipped with this ignitionsystem that it isdesirable to employ a variable resistance at R. Thedifficulty with a fixed resistance is that if'it is high enough to givethe desired'damping' at high speeds-e. g. speeds of approximately 200sparks per second and higher'so as to' prevent missing, the speed atwhich the engine must be driven to start it is increased and thisisundesirable. The difficulty may be overcome by :employing a'specialresistance which increases with engine speed. One way of accomplishingthis is to employ wire, the resistance of which increases rapidly withincrease of temperature, for example wire having a temperaturecoefficient of resistance of about .0045 ohm per ohm per-degreeCentigrade in placev of standard resistance wire having a temperaturecoefficient of resistance of about .0002 When starting an engine with avariable resistance of this sort. the resistance. is cold and has a lowvalue, for example approximately 150 ohms, but as the engine speedincreases the current through the'resistor increases causing atemperature rise and consequent. resistance rise to about 300 or e00ohms, at 3600 R. P. .M. The increased resistance at higher speedsinsures against missing of the engine through ignition failure. I I gAnother wayof accomplishing this result is disclosed in Figure 2 inwhich there is shown'a speed controlled resistance R. which may besubstituted for resistance R of Figure 1. Here resistance R. consists oftwo parts R2 and Rs; 80

indicates a circuit shunting resistance R3 and containing switch 8|which is adapted to'fbe opened in any suitable manner by the operationof centrifugal force as the speed increases thereby increasing theresistance in series'with condenser C. Merely for purposes ofillustration, I have indicated at 82 a conventional type of centrifugal,

governor connected to the switch by link 84 and adapted to openit whenthe governor is'driven at sufiicient speed. Obviously the mechanismcould readily be designed to increase the resistance by several stagesas the speed increases.

The circuit is susceptible of many other modi- With the system herefications. For example, other types of magnetos may be employed, or, ifdesired, a generator not employing permanent magnets may be used. Insome instances it may be satisfactory to employ a battery, inductanceand interrupter in place of the generator as disclosed in the Randolphand Duffendack patent referred to. However, a source of alternatingcurrent is particularly desirable as the reversal in direction ofcurrent flow lengthens the lives of tubes and contacts. While themagneto illustrated is of the type embodying a built in transformer, ifdesired the transformer may be an independent unit or may be omittedentirely, and the primary voltage which appears across the primarycondenser may be applied directly to the tube and high tension coil.

The distributor may be of conventional construction and is preferably ofthe type in which the brushes make sliding contact with the segmentsconnected to the plugs.

The tube is preferably of the construction described and claimed inapplication Serial No. 589,392, filed April 20, 1945, by Ralph S.Mitchel, consisting essentially of low Work function electrodespreferably of a nickel barium composition in an atmosphere of inertgases, preferably a mixture of neon, argon and helium. While aparticular design of tube is illustrated, it will be understood that thedesign is susceptible of considerable variation.

There may be considerable variation in the arrangement of parts on theengine. For example, it has been found convenient in one installation tomount the tubes on the distributor terminals, the distributor itselfforming a part of the magneto assembly. In some instances it may befound satisfactory to employ a single tube rather than one for eachspark plug and in such case the tube would be inserted in series withthe condenser and in advance of the distributor.

Accurate timing is insured by employing an interrupter in the primarycircuit of the magneto. However in some applications it may be foundpossible to dispense with the interrupter and rely upon the action ofthe tube to secure the desired timing.

If desired a separate condenser could be provided for use with eachplug, but this complicates the system and offers little advantage.

It may prove desirable to tune the circuits for optimum impedance,particularly the discharge circuits of the condenser.

I claim:

1. An ignition circuit comprising a source of alternating current, astep-up transformer having its primary in series with the source, adamping resistance and a condenser in series with the secondary of thetransformer, a second stepup transformer, and a discharge circuit forthe condenser comprising a distributor, an electrical discharge tubeadapted to break down upon application of a predetermined voltage, andthe primary of said second step-up transformer, the secondary of saidsecond transformer having a spark discharge device in series therewith.

2. In an ignition system for internal combustion engines of the type inwhich pulsating electric current is supplied from a source to acondenser to cause it to periodically and at a frequency determined byengine speed build up to a voltage sufficient to cause break down of anenclosed spark gap in series with an ignition device to produceignition, a resistance in series with said source and condenser andadapted to damp out oscillations of current upon break down of said gap,and means for increasing said re sistance with increase in speed of theengine.

3. In an ignition circuit, a condenser, means for periodically chargingthe condenser comprising a source of pulsating current and a chargingcircuit between the source and the condenser including a dampingresistance, and a discharge circuit for the condenser comprising inseries a distributor and a plurality of parallel circuits adapt-ed to besuccessively brought into series with the condenser by the distributor,each of said parallel circuits comprising a discharge tube adapted tobreak down upon application of a predetermined voltage and the primaryof a stepup transformer, the secondary of said transformer having aspark discharge device in series therewith, and said damping resistancebeing of such value as to damp out the condenser discharge in sufficienttime to permit re-charging of the condenser between spark discharges.

4. In an ignition circuit, a condenser, means for periodically chargingthe condenser comprising a source of pulsating current and a chargingcircuit between the source and the condenser including a dampingresistance, and a plurality of discharge circuits for said condensereach comprising a discharge tube adapted to break down upon applicationof a predetermined voltage and the primary of a step-up transformer, thesecondary of the transformer having a spark discharge device in seriestherewith, and a distributor for directing discharge of said condenserthrough each of said discharge circuits in succession, said dampingresistance being of such value as to damp out condenser discharge insufiicient time to permit re-charging of the condenser between sparkdischarges.

5. In the circuit as defined in claim 4, said resistance having arelatively high positive temperature coefficient of resistance.

LUCIAN B. SMITH.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

